Active pixel sensor (APS) imaging devices are described in U.S. Pat. No. 5,471,515. These imaging devices include an array of pixel cells that convert light energy into electrical signals. Each pixel includes a photodetector and one or more active transistors formed of MOS technology. The transistors typically provide amplification, readout control and reset control, in addition to producing the electrical signal output from the cell.
APS devices may be fabricating using CMOS technology. An APS sensor, including a pixel array and readout structure, may be provided on the same integrated circuit (IC) chip. The gain stage of the APS sensor may consume a considerable amount of the chip area. However, reducing the size of this area may sacrifice signal gain in the APS sensor.
The readout structure of an APS typically includes a source follower transistor. Use of such a transistor may itself limit the dynamic range of the sensor when reading out the pixel and provide a voltage gain less than unity.